Structuring and catalytic activity of palladium thin layers deposited on the Ni(110) surface

Small amounts of palladium atoms, ranging between 0.02 and 3 monolayers in thickness, were deposited onto a nickel (110) surface and studied by scanni ng tunneling microscopy, first in the as-deposited state and then after ann ealing at a temperature that allowed only surface diffusion to occur. Two d ifferent types of behavior were observed. At low submonolayer coverages pal ladium adatoms form elongated islands along [110] on terraces, and palladiu m and nickel atoms alloy within the surface layer near the steps. At higher coverages very specific chain-like nanostructures form along the [001] dir ection, i.e., perpendicular to the [110] direction bf open rows. In the lat ter case, the surface morphology differs strongly from that of the initial singular (110) surface; this surface restructuring is attributed to the sur face stress that develops when palladium atoms are deposited on the nickel surface. These two kinds of surface morphology can be analyzed with respect to the c atalytic performance, which was tested for the 1,3-butadiene hydrogenation reaction. For low palladium deposits the reactivity is comparable to that m easured for reference surfaces. such as pure Pd(110). However, the catalyti c performance of palladium deposits near or above half a monolayer is impro ved by more than one order of magnitude. We propose to correlate such enhan ced catalytic properties to the presence of very unusual surface sites crea ted, along with the chain-like nanostructures, at strained surfaces. (C) 19 99 Elsevier Science B.V. All rights reserved.